FAILURES IN PRINTING.
It is generally an advantage to work the press in a cool and somewhat damp apartment.
Breakage of the Plate.—Notwithstanding its thickness, the glass plate sometimes flies to pieces on pulling the first impression. If the plate and its foundation are perfectly level the breakage must arise from the presence of some foreign substance having found its way between the two surfaces.
This may occur from such a trivial matter as a grain of sand, perhaps introduced between when moistening the under side of the plate to secure adhesion. Or traces of chromated gelatine may be found attached to the under surface of the plate, either of which causes is quite sufficient to account for a breakage with even moderate pressure. In Collotype the pressure need not be by any means so heavy as in lithographic printing; it should be evenly distributed, and may be moderated by placing the stone or glass plate, or whatever the Collotype plate may be attached to, on some comparatively elastic bedding, which may consist of several layers of printing paper, thin felt, thick indiarubber sheeting, or linoleum, any of which may be used.
Spotted Impressions.—The plate yields unsatisfactory copies covered with white spots resembling drops of water. This most frequently occurs at the commencement of the printing, and is nearly always caused by uneven damping of the surface. Old plates which have been dried very often give perfect negative impressions at first. In such cases, after the removal of the ink by turpentine, soak the plate for a quarter of an hour in water, to which has been added a little glycerine and ammonia (see “Etching Fluid”). Dry the plate with a clean roller covered with cloth or chamois leather—or, as a substitute for the roller, a soft linen cloth may be used—and again roll in with ink which has been thoroughly well distributed on the inking slab.
Too Much Ink is taken by the plate over the whole surface, and notwithstanding slow or rapid rolling, it refuses to leave the high lights. Cause—over-exposure in proportion to the thickness of the layer, which is probably too thin. In such cases washing with dilute solution of ammonia will often improve matters. Another proceeding to be recommended in the case of a plate taking too much ink is to remove all ink by means of a sponge damped in water containing a little petroleum—not benzine or turpentine—and afterwards wash the plate with a sponge containing glycerine and water. It is then ready for immediate printing from, and ought to give about 200 copies without further damping if the press be in a moist situation.
Loss of Half-tone arises from the print layer becoming too moist, when the more delicate half-tones fail to print. Dry with blotting paper, and immerse for five minutes in a bath of alcohol; this will take up the water, and the succeeding impressions will be greatly improved.
Uneven Inking.—It sometimes occurs, particularly in pictures which should show a white ground, that the latter takes more ink in one part than another. This usually arises from an unequal thickness of the sensitive coating. The plate will take most ink where the coating is thinnest, and for this fault there is no remedy.
Ink Refused by the Plate.—Sometimes, after a few impressions have been pulled, the plate refuses to properly take the ink, from the layer being too thick and having been too freely moistened. Further damping should be discontinued and a thinner ink used, which will better take upon the deep-lying shades. This fault will be found most pronounced in the cases of written or printed titles to pictures, and other cases exhibiting extremely disproportionate action of light. Therefore, when there is writing to a picture, it should be shaded somewhat during printing in the printing frame, or if the contrast be very violent it may fail to print entirely.
Flat Prints.—After a number of satisfactory prints have been produced the succeeding ones become flat, this fault will be especially noticeable with thin layers devoid of isinglass, and in cases where the negative has been too flat and lacked contrast. Dampen more frequently with dilute ammonia and glycerine.
Paper Adhering to the Gelatine—Those layers which yield the best and most brilliant impressions, showing plenty of contrast, which have been fully printed under a vigorous negative, most often exhibit a disagreeable tendency to adhere to the paper in the high lights. The greater the proportion of isinglass present in the film the more will this fault manifest itself, particularly with an enamel paper. An application of a diluted solution of ox-gall to the film, the use of a thick well-sized or glazed paper, lighter pressure while printing, and less frequent damping will all tend to diminish this evil. Layers of this nature are extremely liable to damage, and the adhesive portions will soon suffer unless the greatest care is exercised.
Ink Remains upon the Plate.—Through too light or uneven pressure it sometimes happens, especially when using a stiff ink, that the latter will, after the impression has been pulled, partially adhere to the darkest portions of the printing surface, particularly on the edges of plates deeply copied. If this fault is not corrected at once, and the printing allowed to proceed, these portions giving a light, faulty impression will gradually increase in size, and the corresponding portions of the Collotype plate gradually darken. When first observed the whole plate must at once be cleaned with turpentine and again rolled up with thinner ink, a greater and perfectly even pressure being applied. Small faults of this nature occurring on the edges of the plate can be corrected after each faulty impression by means of a small piece of sponge moistened with gum water or with a turpentine rag.
Displacement of the Paper under the scraper will create a double impression, and sometimes wrinkles, the latter invariably leading to the destruction of the plate. By adopting the use of the bridge now used in conjunction with most Collotype hand presses, this fault is avoided, as the paper is only in contact with the printing surface immediately under the scraper.
CHAPTER XII.
Investigations on Collotype.
AUGUST ALBERT[L] is of opinion that it is by no means a matter of indifference whether bichromate of potassium or bichromate of ammonia is used in the preparation of the Collotype plates; on the contrary, he believes that the latter salt materially conduces to interruptions in printing, especially in unsettled weather. The results of his experiments with the two salts in the Collotype process, and of their behaviour in combination with different sorts of gelatine, are as follows:—
To 1000 cc. of water are added ten drops of a saturated solution of chrome alum; this mixture, after having stood for a quarter of an hour, must, if shaken up, show an opalescent appearance, and a slight milky turbidity; if too much chrome alum has been added, the water acquires a greenish colour. The gelatine is now added, and, for about fifteen minutes, kept at a temperature of 134°, when the chrome salt is added, stirring all the while, and the temperature of the water bath raised to 150° to 160°. The temperature and time must strictly be adhered to. The jelly thus prepared should be kept at least ten hours in a solidified state previous to using.
[L] This experimentalist has evidently been confused with the late Joseph Albert, of Munich, by some writers. It may be mentioned that Joseph Albert, whose business is still carried on by his widow, was father of Dr. Eugene Albert, known in the photographic world for his orthochromatic collodion emulsion. August Albert, of Vienna, is of another family.
Hard Gelatine.—Plates prepared with this, show after drying, a shiny, transparent film, more or less according to the quality. Such plates give hard prints, and are soon used up in printing, the finest and even the middle tints becoming weaker, and the shadows gradually coalescing, specially in the case of short exposure and very dense negatives, and they are generally useless after a few impressions have been taken. If ammonium bichromate is used, together with hard gelatine, then, as a general rule, it is impossible to obtain more than one or two prints from each plate. By washing the gelatine, these inconveniences are not at all removed, no matter which of the two salts has been used.
Soft Gelatine.—If this is prepared with potassium bichromate, then the plates will have a dull but rough and untransparent film, which will be found very unserviceable for printing, as the prints will lack vigour; the high lights will tone, and the whole picture appear blurred. If, however, ammonium bichromate is used in conjunction with soft gelatine, the plates will also have a dull appearance, but though they work somewhat better than those prepared with potassium bichromate, they will not be of sufficient durability, the finer tones soon disappearing during printing. Therefore, neither hard nor soft gelatine are suitable for Collotype work, as the printing operations are rendered more difficult. If both sorts are mixed, the results will be somewhat better, but not like those obtainable with middle hard gelatine, which must be accepted as the most suitable kind for Collotype work.
Middle Hard Gelatine, manufactured by Ferd. Friedr. Creutz, absorbs five times its own weight of water at 66°, without signs of disintegration after standing therein for thirty-six hours. If prepared with it and with chemically pure potassium bichromate, the plates after drying have a beautiful matt surface, from which long numbers can be printed without any falling off of the half tints, and, if the printing is done carefully, even the details in the shadows will be maintained to the last. This bichromated gelatine requires also only a very small amount of re-damping, reproduces faithfully the character of the negative, and the jelly can be used even after a period of four days. If, however, the work in hand requires a harder bichromated gelatine—as, for instance, reproductions of black and white—then the addition of a small quantity of ammonium bichromate will have the desired effect; in some cases equal quantities of the bichromates of potassium and ammonium. If ammonium bichromate is used instead of potassium in conjunction with middle hard gelatine, then the plates will acquire a shining, transparent coating, the manipulation of which is very difficult in printing, and which work too hard, being serviceable, at any rate, for reproductions without half-tint. If, however, the middle hard gelatine is washed before use, it will give, when mixed with the chromic salt, transparent films of insufficient durability. Middle hard gelatine should therefore be used unwashed for Collotype work, and mixed with chemically pure potassium bichromate.
The Production of Grain in Collotype plates takes place in the last instant of drying the plates in the drying-cupboard; it depends entirely upon the larger or smaller quantity, and the consistency of the bichromated gelatine used. If the latter is of good quality, all artificial means for the production of grain are not only superfluous, but also disadvantageous. A larger addition of chrome salt generally gives rise to the crystallisation of the salt; chloride of sodium produces plates highly sensitive to moisture, but no grain; a larger addition of chrome alum gives plates which will lack vigour, and the manipulation of which, during printing, will be difficult, as they often will require an “after-etching,” so that even printing is made impossible.
To test the suitability of gelatine for Collotype work, two small portions of the gelatine to be tested may be made up, adding chemically pure potassium bichromate to the one, and ammonia bichromate to the other, and a plate prepared with each solution. If both plates show a dull and untransparent coating, this indicates that it is a soft gelatine; if, on the contrary, both plates appear glossy and transparent, then the gelatine is hard. Should it, however, be middle hard, then the dried film prepared with potassium will be dull, but not rough; that with ammonia, however, shiny and transparent.
CHAPTER XIII.
Collotype in Natural Colours.
A SHORT chapter may be devoted to the various suggested processes for producing Collotype in natural colours, with which are associated the names of Vidal, Albert, and Obernetter. To give a complete resume is altogether impossible, as up to the present time the inventors have only published their method of procedure with very considerable reservations, and notwithstanding the fact that a great deal has been written and published in current literature on the subject, little is actually known. The methods of Vidal and Albert are essentially alike, both being founded upon an opti-chemical basis. By careful registration and printing from three Collotype plates, representing fragments of the same subject upon one sheet of paper, and making use of inks corresponding with the three primary colours, a total effect should be produced which ought, theoretically, to resemble the coloured original. In preparing the three negatives for reproducing the three colour plates a special mode of procedure must be adopted. Each of the three negatives must give a resulting Collotype plate capable of retaining varying amounts of ink when rolled up, according to the predomination in the original of one or the other of the primary colours—red, blue, or yellow.[M] This is effected in the production of the negative for the red plate, by making the exposure through a green glass screen, while the negative intended to produce the plate from which the yellow is to be printed is exposed through a screen of violet.
[M] Pictures produced in the three primary colours are extremely crude. Dr. Vogel has proposed a large number of negatives, using for their production sensitisers corresponding with every region of the spectrum. For example—Napthol blue for red, cyanin for orange, rozin for yellow, and fluorescein for bluish green, the ordinary sensitiveness of the plate being sufficient for blue and violet, the latter, however, being exposed through a yellow screen. The images thus obtained are printed from lithographic stones or Collotype plates, each of which is printed in a colour complimentary to that part of the spectrum to which the particular plate was sensitive. The greater the number of separate images produced in this way the more complete will be the reproduction of the various shades in the original, and the more pleasing the resulting print.
The green screen allowing to pass so few chemically active rays the exposure has to be prolonged to such an extent that gelatine bromide dry plates stained with eosine must of necessity be used.
Obernetter’s method is entirely different, depending more upon mechanical assistance of a non-photographic character in the production of the unlimited number of negatives he uses for producing his printing surfaces, and the results more closely resemble those of chromo-lithography. What particulars Obernetter himself has from time to time communicated are here given. He first produces, by the dusting-on process, as many copies of the original negatives as there are colours to be reproduced. The negative intended to produce the plate from which the blue will be printed is obtained by a long exposure and only slight dusting of the reproduced negative. That intended to reproduce the plate for the non-actinic colours—yellow or brown—should be correctly exposed and dusted in; while that intended for the red should be subjected to a shorter exposure and more vigorous dusting. By retouching, the densities of the various negatives may be modified by strengthening them in parts, other portions are stopped out, and the negatives so manipulated that they will produce plates capable of giving impressions—when printed in their proper colours—giving a facsimile reproduction of the original work. Much will depend upon the artistic qualifications of the operator, who may, by extending the number of plates, finally produce copies of considerable artistic value.
Hösch, of Munich, has patented a process in which the same result is attained in a somewhat different manner. A negative is produced, and from that a Collotype plate from which a number of impressions are taken equal to the number of printing desired or necessary in the finished chromo-collotype picture. Each print has certain portions painted over in a neutral tint, such portions depending upon the colour of the ink in which it is intended to print the corresponding plate. Other portions are stopped out white in the print, or black in the negative, and from the prints so treated a set of negatives are taken; these are utilised for the production of a set of Collotype plates, which, printed from in suitable colours, yield impressions of great excellence.
CHAPTER XIV.
Magic Prints.
TO Professor Husnik (to whom collotypists are greatly indebted for so many unselfish publications), we owe the origination of this interesting method of producing “magic” prints. It is founded on the reaction well known to chemists that two substances, each colourless in itself, when mixed, enter into fresh chemical combinations and produce other substances of intense colour. Herschel and Hunt made use of many of these reactions, both to develop the invisible image, and to give to others, already visible a more agreeable colour or tone. Impressions other than those in fatty ink may be obtained from the Collotype plate—as in the process patented by Edwards—by means of colour, soluble in water, to which the addition of a small amount of gum has been made. In such cases it will not be those portions of the layer which have been rendered insoluble by the action of light which will give the impression (as it would if ink were used) but on the contrary, the unexposed parts, or those which remain soluble. On this account the resulting print will be a negative, and to obtain a positive, the printing must take place under a positive. If a Collotype plate so produced be damped with a linen rag which has been moistened with an aqueous solution of a salt readily absorbed by gelatine, and which will produce in combination with a second solution an intensely coloured compound (the use of the ink roller being entirely omitted), the impression on the printing paper will be almost or even entirely invisible until brought in contact with the second solution, when the picture will be immediately developed and rendered visible. For instance, dampen the plate with an aqueous solution of red prussiate of potash, surface dry it, print upon paper, allow to dry thoroughly, and place upon a solution of protosulphate of iron, when the picture will immediately appear of an intense blue colour. Solutions of bichromate of potassium or nitrate of silver will produce brown images; the former developed with extract of logwood, alazarine or sugar of lead, all produce different colours. Not only is this process interesting, applied in the manner already described, but it is of use in calico printing. Care must however be exercised in the selection of a suitable solution for damping the plate, as many salts would tan the gelatine layer, and such are of course utterly unsuited for the purpose. Among the mordants in common use, salts of tin, alum, iron, &c., may be mentioned as exercising a hardening influence upon gelatine, and therefore useless for moistening the layer. There remain, however, many others which produce no damaging effect upon the plate, and at the same time produce useful colours.
CHAPTER XV.
Photo “Glass” Printing.[N]
[N] This process has never been practised in this country so far as I am aware.—Trans.
THERE remains to be mentioned another variation of the Collotype process known under the above vague title, the distinctive characteristic of which method of preparing the plate is that the negative is taken upon a sheet of glass sufficiently thick to stand the necessary pressure in printing when converted direct into a Collotype plate. After the completion and drying of the collodion negative it is coated upon the film side with the chromated gelatine solution, dried in the drying-box, laid face downwards upon a black velvet surface, and exposed through the back of the glass. The first publication of this method of preparing plates emanated from the State Printing Works at Berlin, and the following is an outline of the method there practised; it is scarcely necessary to state that it is only available for line subjects. The collodion used in the preparation of the negative should be one capable of producing clear, hard negatives entirely free from fog; probably some modification of the dark slide may be necessary to enable it to receive a plate of sufficient strength to stand printing from. The collodion negative being completed it is flooded with the chromated gelatine as applied to Collotype plates, dried, and exposed through the negative until the print layer has assumed a dark brown colour; it is well washed in cold water and damped before printing by an application of gum water containing a little ox-gall; this is done to facilitate the production of perfectly clear impressions. The same end may be attained by the application of dilute ammonia and glycerine. The author has experimented for several years in the same direction with the object of producing heliographic printing plates in copper, for which purpose this method of producing reliefs in gelatine is well suited. From the negative printing plate described, which is treated in all respects as a Collotype plate, impressions may be taken upon transfer paper, and transferred to stone or zinc. Before coating the negative with the chromated gelatine the plate should be allowed to dry. In consequence of printing from the back the lines will always be slightly wider than the original, to meet which difficulty Brandt has proposed putting the negative, when fixed and washed, but still wet, into a weak solution of chromated gelatine, which will, if a suitable collodion has been employed, entirely penetrate the latter. This variation appears worthy of commendation; it is, therefore, here given in the abstract:—“Having produced a negative in the usual manner, on a plate which has been lightly but carefully albumenised, and having fixed and washed the same, it is placed for five minutes in a solution prepared as follows: Allow 45 parts of gelatine to swell in water, and pour off the surplus, add 50 parts of distilled water, and apply sufficient heat to completely liquify the gelatine, adding a solution of 5·5 parts of bichromate of ammonium in 100 parts of water, and heat again to 60° Réamur. With plates of large size the solution may be poured over the plate, and it may be necessary to repeat this operation several times to ensure an even and sufficient coating. Immediately previous to the application of the chromated layer the negative should be rapidly washed over with warm water. A sufficient quantity of the chromated solution must be allowed to remain upon the plate, and the latter warmed in a horizontal position over a spirit lamp. When steam arises the plate has to be held in a perpendicular position for a minute, restored to its horizontal state, and again warmed until the coating of the plate is quite dry, care being taken that at no time the plate is so heated that it becomes unpleasant when placed upon the hand. Place the plate face downwards upon a black support, print until the fine lines are plainly visible, when the plate may be washed and further treated as in Collotype.”
CHAPTER XVI.
Allgeyer’s Collotype Process.
HALF-TONE Negatives must be softer, and possess more half-tone than requisite for silver printing. When examined by transmitted light, they should appear clear in the shadows, and not too dense in the half-tones.
Line Negatives.—Negatives for the reproduction of line subjects need not possess the absolute opacity requisite for silver printing, but the lines should be clear glass, entirely free from fog or deposit.
Stripping the Negative.—The fixed and washed negative is flooded over with a solution of gum arabic in water (1 to 20), to which has been added a few drops of a solution of chrome alum (1 to 60). Allow to dry, retouch with lead pencil, and edge with tallow to prevent overflow, place upon a levelling stand, warm moderately, and coat with a solution of gelatine 100 parts, glycerine 15 parts, alcohol 100 parts, water 400 parts, allow to set, and place against a wall to dry, the coated side outwards, the plate standing upon clean blotting paper. The operation of drying will probably take a day or two, and when quite complete, it is again coated with collodion, and again dried. After allowing to remain in a damp place for two or three hours, cut around the edges with a knife, and strip off the film, which may then be printed from either side, and after use may be stored in a book, the leaves of which consist of smooth paper.
Reproduced Negatives.—Coat a plate with a solution of gum arabic 50 parts, dextrine 50 parts, honey 10 parts, sugar 25 parts, glycerine 5 parts, bichromate of ammonium 15 parts, ammonia 50 parts, water 1,000 parts. After drying, expose under a negative, dust over with graphite, coat with plain collodion, and wash in clean water until all yellowness disappears.
Substratum.—(1.) Water 300 parts, white of egg 300 parts, ammonia 50 parts. (2.) Water 300 parts, gelatine 10 parts, bichromate of potassium 10 parts. Filter both solutions, and mix, taking care that the temperature never exceeds 32° C.; pour upon glass plates heated to 30° C., and dry at a temperature of 22° C. After thoroughly cleaning the back of the plate, place, face downwards, upon a black cloth, and expose to light through the glass till the coating becomes brownish. To test if sufficiently exposed, dampen a portion of the film with the finger, scratch with the nail, and the scratch should stand out perfectly sharp.
The intermediate layer is to ensure the better adhesion to the glass of the print layer proper, and to facilitate the retention of moisture while printing. It consists of gelatine 100 parts, water 1,500 to 1,800 parts, alcohol 150 parts, glycerine 5 parts, bichromate of potassium 5 parts. This is spread over the substratum, allowed to dry, and exposed to diffused light, or allowed to stand for twenty-four hours, well washed, and allowed to dry spontaneously, after which it may be kept for weeks.
The Print Layer consists of water 1,500 parts, gelatine 100 parts, bichromate of ammonium 30 parts, complete solution being effected by means of a water bath; the drying box heated to 45° to 50° C. Place the levelling stand upon a sheet of white paper, carefully dust and accurately level the glass plate, and coat, using 30 ccm. of solution to cover 25 × 35 cm. of surface, avoiding any overflow of solution.
The Washing Water used after printing in the copying frame should not exceed 22° C. in the summer, or fall below 10° to 11° C. in the winter. Washing should be continued until the film is free from colour, and a final rinse should be given under a rose. Allow to dry slowly.
Etching Solution.—Water 500 parts, glycerine 500 parts, chloride of sodium 15 parts, or hyposulphite of soda 15 parts.
Rolling Up.—If the picture take too much ink, wash off with turpentine, carefully wipe with a cloth, and continue the etching. If it take little, wash off the ink with etching fluid, and place in a hardening solution of chrome alum 1 part, water 6 parts, wash, dry, and once more etch for a quarter of an hour.
Varnish for Prints.—Bleached shellac 100 parts, dissolved in alcohol 300 parts, adding ammonia 300 parts, further adding, with constant agitation, 600 parts of boiling water.
CHAPTER XVII.
Practice of Photo-Lithography.
IN photo-lithographic operations, quite a different set of conditions has to be considered as compared with Collotype. An explanation is, of course, not far to seek, as both the chemical and physical properties of the lithographic stone and the exposed chromated gelatine layer are so extremely different. To the better understanding of the operations, the composition of the lithographic stone and the principles underlying the process may be first considered.
Senefelder is a name inseparably connected with lithography, as it was he who, after a continuous and laborious research, discovered the remarkable properties of the Solenhofen limestone, who thoroughly worked out the conditions under which printing with fatty inks could be carried out, and so thorough and conclusive were whose researches that the process as now practised may be said to be actually identical with those of the discoverer. He was by inclination a dramatic author, but failing to find a publisher for the productions of his brain, he conceived the idea of publishing his own works. At a printer’s he acquired the necessary knowledge to perform the work, but lacked the funds to purchase press or type. He then turned his attention to copperplate, made many unsuccessful attempts with an old one he possessed, and when that was unfit for further experiment, extended his operations to the pewter plates of his mother’s dining service, but still without success.
By accident, his next experiments were made upon a stone procured from Cellheim (Solenhofen), and which up to the present had served him as a slab for his ink. For want of paper and ink to write a domestic account for his mother, he casually used the surface of this stone in lieu of paper, using as an ink a mixture of soap, wax, and soot, with the intention of copying upon paper later on. When about to clean the slab, the thought flashed across his mind why not etch the stone with a dilute solution of nitric acid? The experiment was tried, and succeeded beyond expectation. Upon this simple accident has been built up the whole process of lithography. It was only in 1799, after a prolonged and severe struggle, that he was granted protection for his art in Bavaria.
The acid etches away a very thin layer of the unprotected stone, while the writing is not attacked, but left slightly in relief. Senefelder—first with a dabber, and later with a piece of wood covered with cloth—inked in the writing, and so produced impressions, the earliest of which were sheets of music. The etching of a lithographic stone bearing a drawing or transfer in lithographic ink has a twofold action, each of which facilitates the printing. In addition to the slight relief already mentioned, as resulting from the application of the acid, the latter forms upon the surface of the stone, where attacked, nitrate of calcium, which resists and refuses the fatty ink upon the surface of the roller, and if the surface of the stone be kept constantly damp it becomes possible to roll up the drawing, to which alone the ink adheres.
Lithographic stones do not consist merely of carbonate of lime, but contain also silica, alumina, and some iron; the colour is a useful indication of quality, those presenting a bluish grey appearance being most highly prized, while those of a yellow colour are somewhat inferior. The stone may be split into various thicknesses, and prepared for use by grinding either one or both surfaces. They are placed face to face, and the first grinding is completed with coarse sand, for the second grinding a finer sand is employed between the two surfaces, and this is succeeded by a grinding with pumice stone and water, the stone being finally polished with snake stone. The so-called lithographic chalk, with which the drawing on stone is sometimes executed, is composed of finest lamp black (merely added for colour), soap, wax, tallow, and shellac; during the etching, these ingredients undergo a change, and form with the stone a chemical compound of great stability. Since the fatty acids of the soap and tallow-namely, oleic and margaric acid—set free by the nitric acid, combine in the nascent state with the lime to form oleo-margarate of lime. This adheres tenaciously to the stone, and the whole picture may, by the application of turpentine, to all appearances be completely removed from the stone. If the ink roller be again applied, it will be found that even the finest details remain intact, and will reappear of their original strength, the ink only adhering to the oleo-margarate of lime, and not to the surface of the stone, which owing to the etching is wet with nitrate of calcium.
This is the theory of lithographic etching and printing, to which it has only to be added that soap and tallow must always be present in the inks or chalks used in drawing to produce the combinations referred to. Wax and shellac are only present as binding material. Transfer ink, as used in photo-lithography, must be slightly thinned, and this is best done by means of turpentine, and, if necessary, the occasional addition of a few drops of olive oil. Should the ink be too stiff, or be allowed to dry too much upon the transfer paper, the transfer will probably be faulty, as the ink will only partially leave the paper. If the ink remains in a soft condition it will be absorbed very readily by the clean dry litho stone (not etched). Owing to the avidity with which all grease is absorbed by the stone, it is necessary to keep it perfectly free from all such contaminations. To keep the stone in a clean condition while printing it must be etched, kept damp during working, and, on completion or suspension of work, the surface should be coated with a thick solution of gum arabic. Before resuming printing operations, it is of course necessary to remove the gum from the surface of the stone, which is done with a damp sponge.
As an introduction to the practical work of lithographic transferring and printing the following description of the autographic method will be found of interest, especially as it bears some similarity to the more difficult process of photo-lithography, and enables everyone possessing a lithographic press and stones to print their own circulars, price lists, &c.
CHAPTER XVIII.
Autography.
THERE are several methods of transferring from specially prepared, or even any strong well-sized paper.[O] As the latter may be most convenient, proceed as follows: Write with autographic transfer ink, readily procurable from dealers in litho requisites; this will keep for years, and may be used with any clean steel pen. After the writing has dried, damp the back of the paper with dilute nitric acid—1 to 3 of water. When the writing is visible through the back of the paper, the sizing has been acted upon sufficiently; the sheet should be placed in a porcelain dish, washed in several changes of water, and blotted off, leaving it evenly damp without being exactly wet. In the meantime a stone should be adjusted upon the bed of the press, rubbed over with pumice powder, and warmed—this may be easily effected by pouring spirits over the stone and igniting it. A suitable scraper—one which, when pressed against the stone, allows no light to pass between, and which should not be wider than the stone—is adjusted in the scraper box. The screw or other appliance for regulating the pressure is then adjusted. The transfer is carefully and accurately placed upon the warm dry stone, with the writing or drawing face downwards in contact with the polished surface, a few sheets of waste paper are placed over the whole, the tympan (the upper surface of which has been freely rubbed with tallow) is lowered into position, the bed is then pushed, with the stone so covered, under the scraper box, so that the scraper may come down, not upon the extreme edge of the stone, but about four or five cm. from the end, lower the scraper (or, according to the construction of the press, raise the bed) and pull through under gentle pressure. Release the pressure of the scraper, withdraw the bed, and raising the corners of the waste sheets used as backing paper, convince yourself that the transfer is in a proper position and has not moved; replace the covering paper, change the position of the stone so that it passes under the scraper in a reversed position, pass through the press again, this time with greater pressure. Repeat these operations three or four times, on each occasion under steadily increasing pressure; remove the transfer carefully by one corner, and allow the stone to become cold; sponge over with thick gum solution, and in this condition it may be left for any length of time. Great care must be taken that none of the tallow used on the tympan, or other grease, comes in contact with the stone, either by the hand, covering paper, or other means.
[O] Transfer paper is more reliable.
If the transfer should exhibit any signs of weakness while the stone is still covered with plain gum water, take, upon a small linen pad, a little ink thinned either with tallow or a few drops of turpentine; by careful rubbing in different directions the lines will be considerably strengthened. Plenty of gum must be kept upon the surface of the stone during this operation, at the completion of which it may be removed with a sponge and water, and the inking-up completed with the roller and the stone gummed in.
Before printing, the gum has to be removed by water, and the stone rolled up with a moderately stiff ink; it is then etched with gum water rendered slightly acid by a few drops of nitric acid. The best test for the acidity of this solution is to add just so much acid to the gum water that, after its application to the edge of the stone for two or three seconds, scarcely perceptible bubbles of carbonic acid gas will show. This is applied evenly to the drawing with a sponge, and allowed to act for a few seconds.
Should the ink “take” to the stone in places other than the design or writing, rub the spot at once with woollen material moistened with gum water; this will clear away the superfluous ink, but the part should be carefully watched during the printing for any recurrence. After the removal of each impression the stone is damped over the whole surface with a damping cloth before another application of the inking roller.
For the production of perfect impressions avoid contact of the hands with the face of the paper or stone, and care must be exercised that all matters connected with the printing are kept scrupulously clean, any contamination of a fatty or greasy nature especially being strenuously avoided. The paper used in lithographic printing is usually slightly damped by placing between sheets of damp blotting paper; such proceeding is not absolutely necessary, as dry sized paper may also be used.
The above detailed description of the necessary sequence of operations in the Autographic transfer process will assist in understanding that of photo-lithographic transfer, which requires still more care, and the use of a special transfer paper so prepared that it is sensitive to light.